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Field  Columbian   Museum 

Publication  34. 

Geological  Series.  Vol.  I,  No.  4. 


THE    MYLAGAULID^: 

AN    EXTINCT    FAMILY    OF    SCIUROMORPH 

RODENTS 


BY 


Elmer  S.  Riggs,  A.M., 

ASSISTANT    CURATOR,     DEPARTMENT    OF    GEOLOGY. 


Oliver  Cummings  Farrington,  Ph.D., 

CURATOR,  DEPARTMENT  OF  GEOLOGY. 


Chicago,  U.  S.  A. 
March,  1899. 


THE  MYLAGAULID/E;  AN  EXTINCT  FAMILY  OF  SCIUROMORPH 

RODENTS. 


The  genus  Mylagaulus*,  Cope,  was  originally  based  upon  a  sin- 
gle lower  premolar  tooth,  but  later  it  was  supported  by  the  descrip- 
tion of  a  fairly  complete  mandible,  f  From  this  material  two  species 
were  determined,  M.  Monodon  and  M.  sesquipedalis.  Later  the  fam- 
ily name,  Mylagau/idce,  was  proposed  for  these  forms.  To  this 
group  it  is  now  proposed  to  add  the  two  genera  described  below. 


Lateral  view  of  mandible  of  Mesogaulus  ballensis  X  \. 


Dentition  of  same  as  seen  from  above  X  \. 

Mesogaulus  ballensis,  gen.  et  sp.  nov- 

A  rodent  mandible  from  the  Deep  River  escarpments  near  White 
Sulphur  Springs,  Montana,  and  now  in  the  Museum  collection,  shows 
interesting  characters  hitherto  undescribed.  As  suggested  by  Pro- 
fessor Scott,  its  nearest  affinities  are  with  Mylagaulus,  Cope,  though  it  is 
a  more  primitive  form  and  evidently  stands  in  an  ancestral  relation  to 
that  group.  The  name  Mesogaulus  ballensis  is  proposed  for  it  in 
honor  of  Mr.  S.  H.  Ball,  the  collector.  The  characters  distinguish- 
ing this  genus  from  Mylagaulus  are:  First  molar  elongate  antero-pos- 
teriorly  with  enamel  slightly  inflected  externally ;  the  third  molar 
rather  stronger  than  the  second  and  rotund  in  outline.  The  type 
specimen  was  found  in  the  alluvium  at  the  base  of  an  escarpment,  so 
that  its  horizon  cannot  be  exactly  determined.  However,  it  certainly 
belongs  to  the  upper  beds.^ 

•Bulletin  of  the  U.  S.  Geological  and  Geographical  Survey,  vol.  iv.,  No.  2,  May,  1878. 
tAmerican  Naturalist,  Jan.  1883. 

JThe  Mammalia  of  the  Deep  River  Beds.  Am.  Phil.  Soc,  Oct.  1893. 
I8l 


182  Field  Columbian  Museum — Geology,  Vol.  i. 

The  specimen  consists  of  a  single  mandible  from  which  the  angle, 
condyle,  part  of  the  coronoid  process  and  the  tip  of  the  incisor  have 
been  lost.  The  remainder  is  in  good  preservation  and  shows  a  per- 
fect molar  dentition.  The  teeth  are  deeply  set  in  the  alveolus  and 
their  well-worn  crowns  indicate  an  animal  fully  adult.  The  molars 
are  ranged  in  a  line  parallel  to  the  plane  of  the  symphysis,  but  the 
axes  of  the  teeth  themselves  are  inclined  well  inward  at  the  crowns. 
As  in  Mylagaulus,  the  angle  of  the  mandible  is  within  the  plane  of  the 
incisive  alveolus,  but  the  condyle  would  fall  somewhat  without  it. 
The  coronoid  process  is  strong  at  the  base  and  is  directed  outward, 
leaving  a  broad  fossa  between  its  mesial  surface  and  the  posterior 
molars.  At  the  hinder  margin  of  this  fossa  the  dental  foramen  opens. 
Almost  continuous  with  the  anterior  margin  of  the  coronoid  process, 
arising  just  below  the  posterior  third  of  the  premolar  alveolus  and 
extending  downward  toward  the  angle,  is  a  strong  masseteric  ridge. 
The  symphysis  extends  far  back  beneath  the  premolar  alveolus  where 
its  termination  forms  a  prominent  angle  in  the  inferior  outline  of  the 
mandible.  The  ramus  is  concave  on  the  mesial  surface  throughout, 
and  posteriorly  bears  evidence  of  a  deep  fossa.  The  incisor  is  cov- 
ered anteriorly  with  a  thin  orange-colored  enamel.  At  the  alveolus, 
where  it  is  broken,  the  section  is  roughly  sub-triangular.  The  ante- 
rior surface  meets  the  median  one  in  a  sharp  angle ;  with  the  lateral 
surface  it  forms  a  rounded  one  ;  while  posteriorly  the  faces  converge 
to  form  a  single  rounded  surface.  The  premolar,  as  before  men- 
tioned, has  its  crown  much  elongated  antero-posteriorly  and  the 
enamel  slightly  inflected  on  the  lateral  margin.  A  series  of  three 
enamel  fossettes  are  arranged  in  the  median  line  of  the  crown,  with  a 
trace  of  a  fourth  lying  lateral  to  them.  The  anterior  surface  is 
comparatively  narrow,  the  crown  reaching  its  greatest  width  oppo- 
site the  third  fossette,  while  opposite  the  middle  one  occurs  the  exter- 
nal enamel  inflection.  The  second  molar  is  a  small  tooth,  com- 
pressed antero-posteriorly,  and  like  the  third,  entirely  devoid  of  fossae 
or  enamel  inflections.  In  both,  the  enamel  is  absent  from  the  mesial 
surface  and  in  the  first  it  is  noticeably  thinner.  All  are  encased  in 
thick  sheaths  of  cement.  The  last  molar  is  grooved  on  the  mesial 
surface  throughout  its  length,  indicating  the  not  very  remote  coales- 
cing of  roots.  The  other  molars  I  have  not  examined  below  the  alve- 
olus. A  possible  trace  of  a  fourth  alveolus  remains,  but  the  space 
between  the  third  molar  and  the  dental  foramen  is  so  reduced  that  a 
fourth  tooth  could  not  have  developed  in  this  individual. 


The  Mylagailid*: — Rigcs.  1:83 

Ml   AM    Kl    MINTS.  M. 

Length  from  posterior  incisive  alveolus  to  posterior  molar  surface 023 

Length  of  inferior  molar  series 013 

Length  of  diastema 010 

Anteroposterior  diameter  of  premolar 009 

( Ireatest  lateral  breadth  of  premolar 0042 

Greatest  lateral  breadth  of  last  molar 0035 

Depth  of  mandible  at  anterior  molar  alveolus 017 

Protogaulus  hippodus  gen.  nov.  (sp.  Cope.) 

In  view  of  the  later  described  relationship  of  Meniscomys  hip- 
podus* to  the  mylagaulid  phylum,  as  well  as  its  differences  from 
other  members  of  the  genus,  it  seems  desirable  for  the  sake  of 
natural  grouping  to  transfer  the  species  to  the  Mylagaulida.  Upon 
these  grounds  it  is  proposed  to  create  for  it  a  new  genus,  retain- 
ing the  specific  name.  The  generic  characters  thus  distinguish- 
ing Protogaulus  from  Meniscomys  are:  "  Superior  molars  short- 
rooted  with  external  face  plane  ;  inferior  molar  with  a  prominent 
median  transverse  crest  ;"t  premolar  much  larger  than  molars 
and  having  the  crescents  separated  by  a  deep  external  fissure  ; 
inferior  incisor  with  a  wide  groove  on  its  external  face. 

MEASUREMENTS.!  M. 


Length  of  superior  molar  series 

Length  from  base  of  first  superior  molar  to  base  of  incisor. 

Width  between  bases  of  first  molars 

Length  of  first  inferior  molar 

Depth  of  ramus  at  second  molar 


008 
0065 
002 
0033 

0050 


Relationship  :  The  relations  of  Mylagaulus  were  for  some  time 
left  in  doubt,  but  eventually  Professor  Cope  proposed  for  these 
forms  the  family  Mylagaulidce.  As  pointed  out  by  him,  the 
marked  development  of  the  lower  premolar  and  the  characters 
of  its  crown  present  striking  similarities  to  the  dentition  of  cer- 
tain species  of  Hystrix,  especially  H.  refossa.  But  the  complete 
absence  of  roots,  even  in  the  more  primitive  form,  Mesogaulus, 
together  with  the  dissimilar  origin  of  the  angular  portion  of  the 
mandible,  render  the  suggested  relationship  improbable.  All  of 
these  characters  are  essentially  sciuromorph  and  to  that  group  the 
Mylagaulidce  certainly  belong. 

The  presence  of  but  three  molar  teeth  and  the  complete  isola- 
tion of  the  enamel  fossettes  were  offered  as  grounds  for  the  erec- 
tion of   the  family  Mylagaulidtc      If  these  characters  were   pos- 

♦Proceedings  American  Philosophical  Society,  1878  (-79),  p.  67. 
tCope's  Tertiary  Vertebrata,  p.  829. 


184  Field  Columbian  Museum — Geology,  Vol.  i. 

sessed  by  no  other  forms  they  might  be  regarded  as  of  more  than 
generic  importance,  but  in  point  of  fact,  the  dental  formula  is 
the  same  as  that  of  Eucastor  Leidy*  and  the  later  described  Sig- 
mogomphius  Merriam,f  both  of  which  have  true  castoroid affinities. 
As  to  the  second  feature,  the  isolated  enamel  lacunae  are  inde- 
pendently developed  in  several  forms  and  are  doubtless  depend- 
ent upon  the  amount  of  wear  to  which  the  tooth  is  subjected. 
Therefore  this  group  has  not  been  generally  recognized  by  later 
writers,  and  the  genus  has  been  provisionally  included  under  the 
Castoridce. 

Nevertheless  the  Mylagaulida  appear  to  be  a  unique,  and,  until 
now,  isolated,  form.  The  one  prominent  feature  is  the  unusual 
development  of  the  premolar  to  the  exclusion  of  the  posterior- 
lying  teeth.  Associated  with  this  is  the  great  strength  and 
shortness  of  the  mandible,  the  prominence  and  anterior  position 
of  the  masseteric  ridge  and  the  depth  of  the  ramus  from  alveolus 
to  angle.  These  tell  an  unmistakable  story:  Unusual  capacity 
for  crushing  or  grinding  and  the  attendant  specialization  of  the 
premolar  to  perform  the  function  laid  upon  it.  Just  as  in  the 
Carnivora,  the  lower  first  molar,  lying  immediately  anterior  to  the 
insertion  of  the  masseter  muscles,  has  developed  into  the  princi- 
pal shearing  tooth  ;  so  in  these  forms,  the  last  premolar,  stand- 
ing in  the  same  relation,  has  fitted  itself  for  a  crushing  imple- 
ment which,  in  M.  Monodon,  has  reached  the  highest  degree  of 
specialization  known  to  Rodentia.  It  would  be  most  interesting 
to  know  the  food  habits  of  these  unique  animals.  Such  a  ma- 
choire  might  well  have  fitted  them  for  nut-cracking  or  the  crush- 
ing of  hard-shelled  seeds  ;  but  the  abraded,  crowns  of  all  the 
molars,  together  with  their  hyposodont  character  of  growth, 
makes  it  probable  that  the  great  tooth  with  its  enamel  pits  was 
used  largely  as  a  grinding  surface. 

PHYLOGENY    OF    THE    MYLAGAULIDiE. 

Upon  the  problem  of  phylogeny  the  limited  material  thus  far 
known  throws  comparative  little  light.  An  ancestor  of  Mesogau- 
lus  would  be  expected  to  have  a  less  strongly  developed  premo- 
lar with  at  least  one  lateral  enamel  inflection,  to  show  traces  of  a 
third  lower  molar,  and  perhaps  retain  further  evidences  of  molar 
roots.  These  conditions  are  largely  fulfilled  in  the  proposed 
genus  Protogaulus  hippodus.     In  this  form  the  mandible  has  the 

♦American  Naturalist,  July,  1881,  p.  586. 

tBulletin  of  the  Department  of  Geology,  Univ.  of  California,  vol.  1,  No.  13. 


The  Mylagaulid.* — Riggs. 


185 


premolar  considerably  enlarged,  though  the  crown  is  little  elon- 
gate antero-posteriorly.  The  third  molar  is  retained,  but  is 
somewhat  smaller  than  the  second,  and  short  roots  are  present 
throughout.  Cope  describes  the  mandibular  dentition  as  fol- 
lows:* "  The  crowns  of  the  inferior  molars,  in  the  unworn  con- 
dition, terminate  in  two  crescents;  that  is,  in  elevated,  anterior, 
external  and  posterior  borders,  with  a  transverse  ridge  equally 
dividing  the  space  thus  marked  out  and  joining  the  notch  in  the 
external  crest.  This  pattern  resembles  somewhat  that  of  Anchi- 
theriiim.  The  first  inferior  molar  (premolar)  differs  from  the 
others  in  its  superior  size  and  its  having  the  crescents  more 
widely  separated  by  a  deeper  external  emargination.  On  attri- 
tion the  spaces  bounded  by  the  enamel  crests  are  enclosed 
by  the  junction  of  the  extremities  of  these  crests  on  the 
inner  side  of  the  crown.  Further  attrition  results  in  three 
lakes  within  the  crown  and  one  notch  of  the  external  bor- 
der and  two  notches  of  the  internal  border.  The  anterior 
molar  has  two  lakes  in  its  posterior  area  and  one  large  one 
in  its  anterior  area.  In  old  teeth  there  are  successively  one 
and  no  lakes  left  to  interrupt  the  dentine.  The  inferior  incisor 
has  a  wide  shallow  groove  or  concavity  on  its  external  face." 


(a)  Lower  dentition  of  Mesogaulus  X  2.    (b)  The  same  of  Protogaulus  (Mettiscomys,  Cope)  X  4. 
(c)  Upper  dentition  of  Hystrix  refassa  X  2. 


From  this  the  affinities  with  Mesogaulus  at  once  appear.  In 
that  genus  the  lateral  emargination  of  the  premolar  has  almost 
disappeared,  and  the  cleft  is  represented  by  the  deeper  middle 
fossette.  The  large  anterior  lake  described  in  Proiogaulus  is 
replaced  by  a  smaller  one  in  the  later  form;  the  deep  postero-in- 

♦Copes  Tertiary  Vertebra,  p.  828. 


186  Field  Columbian  Museum — Geology,  Vol.  i. 

ternal  cleft  is  traceable  in  the  elongate  third  fossa,  which  is  barely 
separated  from  the  enamel  border.  A  mere  trace  of  a  fourth 
fossette  lying  lateral  to,  and  between  the  second  and  third,  doubt- 
less represents  the  posterior  one  in  the  ancestral  form.  The  true 
molars  have  lost  their  important  characteristics  in  the  process  of 
reduction,  but  even  in  the  earlier  form  the  first  molar  shows  the 
antero-posterior  compression  so  noticeable  in  Mesogaulus,  while 
in  that  form  the  second  molar  retains  something  of  the  diagonal 
elongation  seen  in  the  other,  and  also  a  trace  of  its  median  inter- 
nal flexure. 

The  following  table  will  be  of  service  in  showing  the  distribu- 
tion, both  in  time  and  locality,  of  the  extinct  sciuromorph  rodents. 
It  is  based  upon  Zittel's  classification,  except  in  instances  where 
later  discoveries  have  thrown  additional  light  upon  the  subject: 


The  Mvlagaulid^e— Riggs. 


187 


DISTRIBUTION    OF    SCIUROMORPH    RODENTS 
IN    NORTH   AMERICA. 


Oligo- 
cene. 

MIOCENE. 

Plio- 
cene. 

Pleisto- 
cene. 

V 

V  > 

it 

Q 

c 
£ 
0 

> 
— 
Q 

■?» 

.003 

i 
z 

S 

3 
3 

Q 

■ 
0 

Geomyidae : 

Gymnoptychus  montanus 

X 

"               trilophus 

X 

Heliscomys  vetus. 

X 

Pleuro liens  leptophrys    . 

X 

Entoptychus  crassiramis 

X 

"          pianifrous  . 

X 

Castoridae : 

Steneofiber  nebrascensis 

"          peninsulatus 

X 

"          pansus 

X 

"         gradatus 

X 

"          montanus 

X 

Eucastor  tortus     . 

X 

Castor  fiber 

X 

X 

"       pelicedens 

X 

"       rosina 

X 

Sciuridae: 

A  rctomys  vetus     . 

X 

"         monax 

X 

"        fossilis 

X 

Sciurus  relictis     . 

X 

"       vottmani 

X 

"       calycinus 

X 

"       panolius 

X 

Tantius  lavideus  . 

X 

Meniscomys  liolophus 

X 

"          cavatus 

X 

"          niteus 

X 

Mylagaulidse : 

Protogaulus  hippodus     .... 

X 

Mesogaulus  ballensis      .... 

X 

Mylagaulus  sesquipedalis    . 

X 

•'           monodon      .... 

X 

